1. Field of the Invention
This invention relates to a tire pressure indicator, more particularly to a tunable tire pressure indicator capable of providing a precise tire pressure reading.
2. Description of the Related Art
A conventional tire pressure indicator, as shown in FIG. 1 and 2, includes an elongated hollow casing 10, a valve-actuating member 11, a piston 12, a plug unit 13, a spring 14 and a pressure indicating rod 15.
The casing 10 has upper and lower end portions. The valve-actuating member 11 is mounted to the upper end portion of the casing 10 and has a nozzle 111 that is adapted to actuate an inflating valve member of a tire (not shown) when the valve-actuating member 11 is connected to the inflating valve member, thereby discharging air from the tire into the casing 10. The piston 12 is mounted movably in the casing 10 adjacent to the valve-actuating member 11 and has a diameter-reduced protrusion 121 projecting downwardly from a bottom side thereof.
The plug unit 13 is mounted in the lower end portion of the casing 10 and includes a cylindrical body which has a diameter-reduced lower portion 131 and a passage 132 of rectangular cross-section. A mounting unit includes a pair of spaced mounting arms 133 which project from a top side of the cylindrical body and which confine a channel communicated with the passage 132. After the cylindrical body has been inserted into the casing 10, the bottom end of the casing 10 is folded inwardly to clip tightly the lower portion 131 of the cylindrical body in order to fix the plug unit 13 in the casing 10. The spring 14 is provided in the casing 10, and has its upper end sleeved on the protrusion 121 of the piston 12 and its lower end sleeved on the mounting arms 133 in order to maintain movement of the spring 14 along a desired direction. The pressure indicating rod 15 is disposed within the spring 14, and has a lower end portion extending slidably through the passage 132 of the cylindrical body and an upper end portion provided with a block 151 to retain the pressure indicating rod 15 within the casing 10.
When the valve-actuating member 11 is coupled with an inflating valve member of a tire to open the inflating valve member, air from the tire can drive the piston 12 against action of the spring 14 in order to push the pressure indicating rod 15 to extend out of the casing 10. Thus, a tire pressure reading can be obtained on the exposed lower end portion of the pressure indicating rod 15 in response to pressure entering the casing 10 via the valve-actuating member 11.
The conventional tire pressure indicator has several drawbacks. For example, when the spring 14 within the casing 10 of the conventional tire pressure indicator experiences fatigue after long-term use, a precise tire pressure reading cannot be obtained from the conventional tire pressure indicator. In addition, during manufacture, when the conventional tire pressure indicator is tested to have an error that is not within a tolerable range, this product has to be dismantled manually to replace the spring 14.